In the garden under study, 48 taxa were identified during the research period. Given that the species found in only a single year are 23% of the total number, and that two new species were observed in the fourth year of monitoring, it is likely that the total number of species which could be found in this garden is somewhat higher than 48. The expected increase may result from missing species during the research period, as well as changing species distribution.

The garden under study shows a large variation of species between years. Only 35% of the species were found every year. Such variation in hoverfly presence has also been observed by de Groot et al. (2022) in urban forest habitats. The variability can be caused by migration behaviour (Ball and Morris 2000; Reemer et al. 2009; Speight 2017; Wotton et al. 2019; Gatter et al. 2020). The absence in some years of species such as Scaeva pyrastri and Helophilus trivittatus may be caused by the fact that these are migratory species without permanent breeding populations (Ball and Morris 2000). Another possible explanation for the observed variation includes differing annual abiotic or biotic conditions in the area (Davies et al. 2009; Popov et al. 2017; Schirmel et al. 2018), which could point to an indicative value of hoverflies in gardens. Additionally, the variation may also be influenced by the research method of photographic capture. The variation of species was not found by Owen and Gilbert (1989); they reported that hoverflies in gardens show a rather stable population.

Some species in the garden were found in only one year and ones can be designated as locally ‘rare’: Dasysyrphus albostriatus, Epistrophe melanostoma, Eupeodes latifasciatus, Helophilus hybridus, Pipiza festiva, Pipiza noctiluca, Scaeva selenitica, Volucella pellucens, Xanthandrus comtus and Xanthogramma pedissequum. Species which were observed every year and frequently can be designed as locally ‘common’: Episyrphus balteatus, Eristalis pertinax, Eristalis tenax, Helophilus pendulus (Fig. 8), Meliscaeva auricollis, Neoascia podagrica and Syritta pipiens. The above mentioned ‘rare’ species were not found in rural or urban gardens in Sweden; the ‘common’ species found in rural gardens in Sweden showed different results, with the exception of Episyrphus balteatus (Persson et al. 2020).

Figure 8. 

Helophilus pendulus (Linnaeus), a common hoverfly in the studied garden.

No non-native species were found in the garden, suggesting that a significant density of non-native flora in the investigated garden does not affect the presence of non-native hoverflies. According to Salisbury et al. (2015), non-native flora in gardens can extend the flowering season and provide additional food resources for hoverflies. On the other side, hoverflies may be more abundant on native flora (Smith et al. 2006a; Salisbury et al. 2015).

As expected, most of the species found in the garden are common or relatively common. The importance of common species for biodiversity is sometimes undervalued. While much research rightly focuses on rare species, common and thus mostly more abundant species should also be investigated for their role in biodiversity. In most biodiversity indices (Fisher et al. 1943; Simpson 1949) both species richness and abundance play important roles in population constants.

Gardens are possibly, in terms of population density, comparable to other small areas such as small woodlands. A broad European study by Valdéz et al. (2019) showed that small woodlands do not harbour many rare species but support high population densities, providing essential ecosystem services.

In the studied garden, species typical for open habitats were found (e.g., Cheilosia albitarsis, Eristalis arbustorum, Helophilus trivittatus, Melanogaster hirtella, Melanostoma mellinum and Sphaerophoria scripta) as well as species typical for deciduous forest (e.g., Epistrophe melanostoma, Eristalis nemorum, Eristalis similis, Pipiza festiva, Volucella bombylans and Xanthandrus comtus. Gardens tend to have an environmental structure similar to open woodland habitat, in terms of differentiated vegetation levels and patterns of shaded and sun-lit areas. Therefore, gardens can inhabit representative species of open woodland habitat. There is a relationship between the hoverfly presence and forest edges (Sjödin et al. 2008; Bortolotto et al. 2016; Schirmel et al. 2018), and thus the hoverflies in gardens may be considered as a natural community for forest edges.

There are several arguments why hoverflies could make suitable indicators for biodiversity and garden health in citizen science research. First, hoverflies are widely represented in gardens and thus accessible to a large public (Owen and Gilbert 1998; Persson et al. 2020). Second, their body size and clear patterns make most species easy to identify by photographic capture and biometrical identification, unlike many other families of Diptera (such as Chironomidae, Cecidomyiidae and Chyromyidae). This method is also an animal-friendly method of monitoring. Third, many people find hoverflies empathetic because of their aesthetic qualities due to the contrasting colours, their characteristic way of flying and the fact that they are non-biting. Fourth, hoverflies are important and economically valuable pollinators for vegetables and fruits (Jauker and Wolters 2008; Calle 2019; Cook et al. 2020). Fifth, numerous studies demonstrate the indicative value of hoverflies (Sommagio 1999; Smith et al. 2006a; Sjödin et al. 2008; Popov et al. 2017; de Groot et al. 2022). There is already some information on species able to serve as indicators of specific micro-habitats in gardens such as the presence of dead wood by the saproxylic Xylota segnis (van Steenis 2023) and the percentage of built space by Melanostoma scalare (Bates et al. 2011). However, for the purpose of developing a Garden Health Index, this specific habitat information is still limited. Disadvantages of using hoverflies as indicators include the migratory behaviour of some species (Ball and Morris 2000; Speight 2017), their possible insensitivity to heavy metals (Mielczarek et al. 2021) and their sensitivity to ambient temperatures (Aguirre-Gutiérrez et al. 2017).

A well-described and easy to repeat method of recording, and more quantitative data about micro-habitat in gardens and the autecology of hoverflies, are needed to develop a reliable Garden Health Index. A lot of information can be obtained through citizen science projects. A study of mosquitoes in Germany shows that reliable entomological data can be efficiently obtained through citizen science (Pernat et al. 2021). Additional advantages of citizen science projects include positive impact on the conservation behaviour of participants (Green et al. 2023) and in the provision of easier access to data from private properties, which are often inaccessible to most research methods (Dickinson et al. 2010). Citizen science can provide tremendous insight into the biodiversity of hoverflies and measures to increase insect diversity in gardens and backyards.